Wash and dry all vegetables except for onions. Cut potatoes into 1-inch
thick lengthwise wedges. Steam wedges on a rack over boiling water
(covered) for 10 minutes to pre-cook potatoes. Remove potatoes from steam
and pat dry with paper towels. Prepare remaining vegetables- cut eggplant
lengthwise into 1 -inch thick slices; cut zucchini and yellow squash in half
lengthwise; seed and quarter green and red peppers. Place potatoes and all
other vegetables on baking sheets and brush with olive oil to prevent
sticking during grilling. Prepare grill. When grill is ready, lightly oil
the grill tray. Place eggplant, squash and mushrooms on the grill crosswise
so they don't fall through the grill tray. Place the tomatoes and the
peppers in a wire grilling basket or on a grilling tray suitable for small
foods so they don't fall through into the grill. Place garlic heads
directly on grill. With the grill lid closed, grill all vegetables for 5
minutes. With tongs and a metal spatula, turn the vegetables and grill the
other side, with the lid closed, for another 5-6 minutes. If all vegetables
don't fit on the grill, cook in batches. When vegetables are cooked,
arrange on a large serving platter (slice Portobello mushrooms if they're

large). Halve the grilled heads of garlic crosswise, and squeeze out the
soft pulp into a small bowl. Whisk together the balsamic vinegar and the
garlic, then pour over the vegetables. If not using garlic, just drizzle
vinegar over vegetables. Sprinkle vegetables with the fresh thyme leaves or
chives, if desired. Serve salad warm or at "picnic" temperature-no need to
chill.